Back To CourseCLEP Biology: Study Guide & Test Prep
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Angiosperms are flowering vascular plants. They are the most common type of plant and make up over 90% of the plant species on Earth. The name 'angiosperm' comes from two Greek words: 'angio,' which means 'container,' and 'sperm,' which means 'seed.' When we combine the terms, we get 'seed container,' which is a very good description of angiosperms. While angiosperms are best known as flowering plants, they are also known for the protective container, or fruit, which surrounds the seed.
Examples of angiosperms range from small wildflowers to large oak trees. Angiosperms can either be unisexual or bisexual, meaning that some flowering plants contain only male or female structures, while others contain both male and female structures. We already know that many plants go through a life cycle that alternates between diploid and haploid, but let's review some basic aspects of this before looking at how angiosperms, or flowering plants, go through this process.
Alternation of generations is a life cycle that includes both diploid and haploid multicellular stages. Remember that 'diploid' means 'two sets of chromosomes' and is commonly abbreviated as 2n, where the n stands for chromosomes. In diploid cells, one copy of the chromosomes comes from each parent.
For example, in humans, you get one copy of chromosomes from your dad and one copy of chromosomes from your mom. The same idea is found in plants. Each diploid cell contains one copy of chromosomes from the male parent and one copy of chromosomes from the female parent. 'Haploid' means 'one set of chromosomes' and is commonly abbreviated as n because there is only one copy of the chromosomes.
Let's look at a diagram of the basic idea of alternation of generations.
We can see in this diagram that the life cycle is broken into n on the top and 2n on the bottom. Remember that the gametophyte contains haploid cells and that the sporophyte contains diploid cells. Previously, we also used the Garblinx to illustrate the oddity of this. Remember that the diploid, or 2n, organism looks like this.
However, when it moves into the haploid stage, the Garblinx looks completely different! Two of these haploid organisms will get together and mate in order to produce a new 2n organism that looks similar to our first Garblinx. This Garblinx will then eventually produce a new haploid organism, and so on.
Now that we have reviewed the basic aspects of this life cycle, let's look at how angiosperms go through the cycle. We will start with the haploid stage as we did previously and then move into the dominant diploid stage. As we move through the different stages in this alternation of generations, we will refer back to this diagram illustrating how angiosperms reproduce.
The first structures in the haploid stage are the microspore and the megaspore. The microspore will develop into the male gametophyte and is much smaller than the female spore. The male gametophyte in angiosperms is the pollen and develops in the anther of the flower. You may remember from learning about the structure of flowers that anthers are the top part of the male stamens. We can see the anther in this diagram.
The other potential start for the haploid stage is the megaspore, which will develop into the female gametophyte and is much larger than the male spore. The megaspores will eventually become the multicellular female gametophyte, which is the ovary. Again, you may remember from learning about plant parts that the ovary is part of the female structures in the flower. We can see the ovary here in this diagram.
Once the gametophytes have fully developed, they will produce gametes. The male gamete is sperm, and the female gamete is an egg. The mature male gametophyte, the pollen grain, develops the sperm, while the mature female gametophyte, the ovary, develops the egg. Pollination will occur when the pollen grain lands on the female structures in a flower. However, until fertilization occurs - the union of the egg and sperm - the cells remain in the haploid stage.
As with most plant life cycles, the diploid stage starts once an egg has been fertilized by a sperm. However, in angiosperms, there is a twist to this normal fertilization known as double fertilization. Rather than just containing one sperm cell, each pollen grain contains two sperm cells. One of these haploid sperm cells fertilizes the haploid egg, making a diploid embryo.
However, the other sperm cell fuses with a part of the female gametophyte, which has two nuclei. This combination of the haploid sperm cell with a double nucleus creates a cell that actually has three copies of genetic information. This 3n structure goes through mitosis in order to develop into the endosperm. The endosperm is a great food source for the developing embryo. The developing embryo will use this source of food and eventually develop into a mature sporophyte.
The mature sporophyte will grow and eventually produce structures for reproduction - namely flowers. Remember that some angiosperms will contain either male or female parts, while others will contain both. Regardless of this, the dominant diploid stage of the life cycle will produce flowers, which are specialized structures for reproduction. The female parts of the flower will produce megaspores, and the male parts will develop microspores, continuing this cycle.
Angiosperms are unique plants because they produce protected seeds. This alternation of generations in flowering plants, such as oak trees and wildflowers, means that there are multicellular stages that are haploid and diploid. We first looked at the haploid stage, which starts as either a male or a female spore that develops into a gametophyte. The male microgametophyte is contained in pollen grains, and the female megagametophyte is found in the ovary. Eventually, the pollen grains will land on the female flower structures and fertilize the egg.
Remember that in angiosperms a process known as double fertilization occurs. In this process, one sperm cell fertilizes the egg, making a new diploid embryo that will eventually develop into a seed and then a mature sporophyte. However, a second sperm cell fuses with a double nuclei cell, creating a cell that now contains three copies of genetic information. This endosperm becomes the food source for the developing embryo. Once mature, the sporophyte will eventually produce flowers in order to continue the process of sexual reproduction via alternation of generations.
At the end of this lesson, you will be able to explain what happens during alternation of generations in angiosperms, including the unique process of double fertilization.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 224 lessons